A traditional telephone system provides an ability for users with modems to connect to a Remote Access Server (RAS) for digital communications over standard voice lines. This same capability is provided by Voice-Over-Internet Protocol (VoIP) gateways (GW) through the use of a Modem Pass Through (MPT) or voice band data (VBD) mode. VoIP gateways also provide telephone voice services replacing traditional telephony switches.
During a call set-up procedure, an originating gateway (OGW) and terminating gateway (TGW) exchange capabilities via a call control protocol (International Telecommunications Union (ITU) H.323 or Media Gateway Control Protocol (MGCP)) and negotiate a voice encoder/decoder (codec) that is supported by both gateways. Some complex codecs, such as ITU G.729, are supported and most commonly used by VoIP gateways for bandwidth saving purposes. These complex codecs work well in most voice-only calls. However, due to the compression algorithm of complex codecs, VBD calls experience problems, including signal distortion and call hang-ups. Therefore, in voice mode, once a VoIP gateway detects a modem/fax Answer Back Tone (ABT), it switches into VBD mode, which uses a low complexity codec, such as ITU G.711, thereby preventing VBD signal distortion and providing reliable VBD call connections.
During the voice-to-VBD mode change, the OGW and TGW communicate with each other using peer to peer messages via a packet switched network, such as an Internet Protocol (IP) network. There are several such protocols for this, including one that uses peer-to-peer messages that carry a Named Signaling Event (NSE) packet with an Event ID defined by a manufacturer or service provider. The NSE packet uses an Network Terminating Equipment (NTE) format defined in ITU Request for Comments (RFC) document 2833 (“RFC 2833”).
RFC 2833 is an IETF “standards track” proposal for carrying Named Signaling Events as RTP packets. According to this standard, the RTP Payload Format for Named Telephone Events includes an Event ID indicating the Particular named telephone event, as well as a volume and a duration for each DTMF digit.
All of the work during a VBD mode switch, including modem/fax answer back tone detection, peer-to-peer message exchange, and the mode switch, introduces time delay. Therefore, if the original voice call is running in a complex codec other than G.711, a certain duration of modem/fax answer back tone may be distorted and transmitted to the client side before the VBD mode switch is completed. This may cause some client modem hang-ups due to the unrecognized modem answer back tone, thus lowering the success rate of VBD calls.
One current solution is to apply a notch filter to block 2100 Hz modem Answer Back Tones so that the tone leakage (i.e., unwanted tones passing through to the client side) does not exceed 50 msec when a call is running on a complex codec other than ITU G.711.